1. Field of the Invention
The present invention is directed in general to magnetic resonance tomography (MRT) as employed in medicine for examining patients. In particular, the present invention is directed to a magnetic resonance apparatus as well as to a method for the operation thereof of the type wherein a 3D field map, for example according to German PS 198 44 420, is employed in combination with the known two-point Dixon method.
2. Description of the Prior Art
Magnetic resonance tomography is a tomographic method for medical diagnostics that is mainly distinguished by a high contrast resolution capability. Due to the excellent presentation of soft tissue, magnetic resonance tomography has developed into a method that is often superior to x-ray computed tomography. Magnetic resonance tomography is currently based on the application of spin echo sequences and gradient echo sequences that enable an excellent image quality with measuring times on the order of magnitude of minutes.
In the presentation of the tissue of patients, however, artifacts that arise from the influence of the chemical shift occur at the boundary layers between fat and water. The phenomenon referred to as chemical shift is the property that the resonant frequency shifts slightly proportional to the field strength dependent on the type of chemical bond in which the nucleus is situated. Due to its concentration in the human body, primarily hydrogen nuclei of free water and of fat contribute to the image. Their relative resonant frequency difference amounts to approximately 3 ppm (parts per million). As a result, a modulation of the signal intensity dependent on the echo time TE occurs given the employment of spin echo sequences and gradient echo sequences.
Such artifacts must be avoided since they can lead to an incorrect diagnosis. It is therefore a problem which persists in the field of magnetic resonance tomography to provide a magnetic resonance tomography apparatus and a method for the operation thereof wherein the artifacts due to the chemical shift between a first spin ensemble, for example water, and a second spin ensemble, for example fat, are diminished or avoided.
The original publication of W. T. Dixon presented a method that achieves a separation of the fat and water images with two echoes (gradient or spin echoes). This shall be described in brief below.
Immediately after a 90xc2x0 excitation pulse is emitted into a subject in, the magnetization vector of the water protons MW and the magnetization vector of the fat protons Mf point in the same direction. This condition, however, does not persist since the water protons precess 3 to 4 ppm faster than the fat protons in the uniform magnetic field. The manner by which the magnetization of the water protons and that of the fat protons disperse over time can be seen in the laboratory system (FIG. 2a). This difference amounts to approximately 50 Hz given 0.35 T. As shown in FIG. 2b, the total magnetization MT is the vector sum of water and fat magnetization. FIG. 2a refers to a reference system that rotates with the frequency of the water protons.
FIG. 2c shows that the total magnetization MT initially exhibits a maximum when the water and the fat magnetization point in the same direction but soon passes through a minimum when the water magnetization and the fat magnetization are anti-parallel (oppositely directed).   t  =            1              2        ⁢                  (                                    v              w                        -                          v              f                                )                      =    a  
wherein t is time, vf is the fat proton frequency and vw is the water proton frequency. The time a is of great significance since the exposure of an imaging sequence at time t=a supplies an image in which the brightness of the pixels is dependent on the difference between fat magnetization and water magnetization. An exposure at t=O1, i.e. when fat and water magnetization are directed parallel, yields an image wherein the sum of fat magnetization and water magnetization is presented.
The sum of and the difference between the two images is of critical significance: the sum yields a water image, the difference yields a fat image.
The method that has just been presented has, however, a great disadvantage: it assumes that the basic field B0 is absolutely homogeneous. Inhomogeneities of the basic field that in fact exist lead do not allow the components to be unambiguously separated.
The 3-point Dixon method was proposed as a possible expansion, this being described by G. H. Glover and E. Schneider in the article xe2x80x9cThree-Point Dixon Technique for True Water/Fat Decomposition with B0 Inhomogeneity Correctionxe2x80x9d, Magnetic Resonance in Medicine, Vol. 18, pp. 371-383, 1991, and being currently widely applied, however, problems continue to exist: Three instead of two images with different phase shift must be registered, which significantly lengthens the measuring time and limits the application of the sequences. Moreover, a phase unwrap of the respective measurement matrix must be implemented, this itself being affected by problems (phase unwrapping of 2D data is not trivial).
It is an object of the present invention to provide a magnetic resonance tomography apparatus and a method for the operation thereof that enable the acquisition of pure fat and water images in a simpler way, taking the basic field inhomogeneities into consideration.
This object is inventively achieved in an apparatus for processing and presentation of a nuclear magnetic resonance tomography measured image, having a radio-frequency system with which a 3D field measurement is implemented for obtaining 3D field data with respect to the inhomogeneities of the basic field. A processing system reconstructs an image from the acquired image data and which interpolates 3D field data over the correspondingly selected slice obtained with the radio-frequency system, and corrects the phase of the imaging data on the basis of the residual phase offset data respectively obtained by means of the interpolation.
Further, shim coils are advantageously provided that are driven by a shim power supply in order to homogenize the B0-field for the slices selected for the following imaging measurementxe2x80x94preferably over the entire homogeneity volumexe2x80x94by suitable application of current to the shim coils. The homogenization is effective to a low order, preferably first order, of the basic field.
The processing system calculates the current for the shim coils on the basis of first 3D field data obtained with the radio-frequency system.
Given a corresponding current applied to the shim coils, the radio-frequency system picks up a new basic field that is non-uniform at a higher order by means of a renewed 3D field measurement, and the processing system interpolates the second 3D field data obtained in this way for a basic field that is non-uniform at a higher order and employs these for the phase correction of the imaging data.
The processing system computationally corrects the first 3D field dataset on the basis of the calculated applied current and the characteristics of the shim coils, and employs the second 3D field data obtained in this way for a basic field that is non-uniform in a higher order for the phase correction of the imaging data.
By means of a spin echo sequence or by means of a gradient echo sequence, further, the radio-frequency system can implement a number of measurements having different relative phase positions of the spin ensembles relative to one another.
The relative phase of the spins of the first and the second spin ensembles is usually different.
In the case of two echoes, the respective spins of the first and the second spin ensembles are advantageously in phase in the first measurement and anti-phase in the second measurement.
Further, the processing system can generate a pure image of the first or of the second spin ensemble by means of the addition and/or subtraction of the first and second datasets.
The first spin ensemble represents water and the second spin ensemble represents fat.
The processing system can be formed by an image computer and a system computer.
The present invention also is directed to a method for the operation of a magnetic resonance tomography apparatus having the following steps:
3D field measurement over a measurement volume, preferably over the entire homogeneity volume;
obtaining 3D field data with respect to the inhomogeneities of the basic field;
reconstructing an image from the acquired imaging data;
interpolating the 3D field data over the correspondingly selected slice;
and
correcting the phase of the imaging data on the basis of the residual phase offset data respectively obtained by interpolation.